如何使用Lodash按条件合并两个对象数组，保留array1全部元素

_.unionBy 会对指定字段做全局去重，每个字段值仅保留第一个出现的元素，无法满足保留重复元素、仅抵消对应数量同name元素的需求，你可以通过_.countBy+_.filter组合实现：

const array1 = [{name: "1"}, {name: "2"}, {name: "3"}, {name: "4"}, {name: "4"}];
const array2 = [{name: "1"}, {name: "5"}, {name: "3"}, {name: "4"}, {name: "4"}, {name: "4"}, {name: "1"}, {name: "1"}, {name: "2"}];

// 统计array1中每个name的出现次数
const nameCount = _.countBy(array1, 'name');

// 筛选array2中需要追加的元素：同name元素抵消完array1的计数后剩余的部分
const filteredArray2 = _.filter(array2, item => {
  if (nameCount[item.name] && nameCount[item.name] > 0) {
    nameCount[item.name]--;
    return false;
  }
  return true;
});

// 拼接得到最终结果
const result = [...array1, ...filteredArray2];

最终输出的result和你期望的结果完全一致：

[{name: "1"}, {name: "2"}, {name: "3"}, {name: "4"}, {name: "4"}, {name: "5"}, {name: "4"}, {name: "1"}, {name: "1"}]

内容的提问来源于stack exchange，提问作者sifho